1) Field of the Invention
The present invention relates to a technology for optical scanning in an image forming apparatus.
2) Description of the Related Art
An optical scanning unit is widely known due to its association with a laser printer etc. In general, the optical scanning unit is constructed to function in the following manner. A light beam from a light source is caused to be deflected by a light deflector. The deflected light is focuses towards a scanning surface by a scanning-imaging optical system that includes components such as an fθ lens and a light spot is formed on the scanning surface. The light spot is scanned optically on the scanning surface. This optical scanning is called as main scanning. The scanning surface is a photosensitive surface of a photoconductive photosensitive medium of a photosensitive drum and a photosensitive belt etc.
An image forming apparatus that includes four photosensitive drums arranged in a direction of transporting of a recording paper to form an image for each color component is known as an example of a full-color image forming apparatus. In such an image forming, the image forming apparatus includes a plurality of light sources, which are provided corresponding to each of the photosensitive drums. Light beams demodulated according to an image signal for each color component are emitted from the plurality of light sources. The light beams emitted are subjected to deflection scanning by one light deflector. Latent images are formed by exposing simultaneously, on each of the photosensitive drums by the scanning imaging optical system corresponding to each photosensitive drum. The latent images are visualized in a developing unit by using developers of different colors such as yellow, magenta, cyan, and black. The visualized images are transferred on the same recording paper by superimposing one after another and fixed, thereby obtaining a color image.
Thus, an image forming apparatus in which a two-color image, a multi-color image, or a color image is obtained by using two or more than two combinations of optical scanning units and photosensitive drums is known as a tandem image forming apparatus.
An apparatus in which a plurality of photosensitive media shares a single light deflector is known as such a tandem image forming apparatus. The following are various types of apparatuses known in which the single light deflector is used jointly.                (1) Plurality of optical scanning units corresponding to a plurality of photosensitive drums—A method of scanning by using the same number of optical scanning units as the photosensitive drums, and synchronizing the light deflectors for all the optical scanning units has been disclosed in, for example, Japanese Patent Application Laid-open Publication No. 2001-305826.        (2) Scanning in opposite directions in which light beams are incident from both sides of the light deflector and deflection-scanning is performed by dividing into two directions—A method in which a plurality of light beams, which are substantially parallel and separated apart in a secondary scanning direction, is incident on the light deflector and the scanning is performed by a plurality of scanning optical elements arranged in the secondary scanning direction corresponding to the plurality of light beams has been disclosed in, for example, Japanese Patent Application Laid-open Publication No. H9-54263, Japanese Patent Application Laid-open Publication No. H11-157128, and Japanese Patent Application Laid-open Publication No. H9-127443.        (3) One side scanning in which light beams are incident from one side of the light deflector and the deflection scanning is performed in one direction—A method in which a plurality of light beams heading towards different scanning surfaces pass through a first optical element L1, a second optical element L2, and a third optical element L3 is provided for each optical beam that heads towards each of the scanning surfaces has been disclosed in, for example, Japanese Patent Application Laid-open Publication No. 2001-4948, Japanese Patent Application Laid-open Publication No. 2001-10107, and Japanese Patent Application Laid-open Publication No. 2001-33720.        
Thus, if the light deflector is used jointly for the plurality of light beams reaching the plurality of scanning surfaces, the number of light deflectors can be reduced. This enables to make an optical scanning unit and an image forming apparatus in which the optical scanning unit is used, compact in size and to reduce the cost.
Recently, in an optical scanning unit of a color image forming apparatus, a technology that uses an oblique incidence optical system as a single light deflector to reduce cost has been known (see, for example, Japanese Patent Application Laid-open Publication No. 2003-5114). In the oblique incidence optical system, light beams are allowed to be incident on a deflecting and reflecting surface of the light deflector at an angle in the secondary scanning direction. In the oblique incidence optical system, after the plurality of light beams is deflected and reflected at the respective deflecting and reflecting surfaces, the beams are separated by a reflecting mirror to individual beams and guided to scanning surfaces, which are surfaces of photosensitive drums corresponding to each of the light beams. At this time, the angle of incidence of each beam in the secondary scanning direction, or in particular, an angle of oblique incidence on the light deflector is set to an angle that enables to separate each light beam at the mirror. By using this oblique incidence optical system, it is possible to secure a distance between adjacent light beams in the secondary scanning direction which can be separated by the mirror without increasing a size of the light deflector, i.e. without using a multi-stage and thick polygon mirror.
Further, according to the oblique incidence optical system, if a case of using the polygon mirror (a rotating polygon mirror) as a light deflector is taken into consideration, in a method of incidence that is used normally, it is difficult to cause the light beams from the light source to be incident towards an axis of rotation of the polygon mirror. Although, it is not impossible to cause the light beams from the light source to be incident towards the axis of rotation of the polygon mirror, when the light beams are caused to be incident towards the axis of rotation, if an attempt is made to secure a required angle of deflection, each of the individual deflecting and reflecting surfaces become extremely big. Due to this, in the method of incidence that is used normally, the size of the polygon mirror cannot be reduced. Moreover, this gives rise to considerable amount of the so called sag, and the sag that is developed is asymmetrical to an image height: 0. Further, as the size of the polygon mirror becomes bigger, a large amount of energy is required for high-speed rotation and when the polygon mirror is rotated at a high speed, there is a loud wind noise and the size of a sound-proofing unit has to be made bigger.
Whereas, in the oblique incidence optical system, the light beams from the light source can be caused to be incident towards the axis of rotation of the polygon mirror, the size of the polygon mirror can be reduced and the wind noise when the polygon mirror is rotated is low. Therefore, the oblique incidence optical system is suitable to be used for high speed. Since the size of the polygon mirror can be reduced, the amount of sag that is developed is small, and the sag that is developed can be made to be symmetrical with the image size: 0. Therefore, it is easy to make a correction.
However, on the other hand, in the method of oblique incidence, bending of scanning lines is a big problem. An amount of bending of scanning lines that is developed varies depending on the angle of incidence of each light beam in the secondary scanning direction. A latent image that is drawn by each light beam on each photosensitive drum is developed by a toner of the corresponding color and then superimposed and visualized. When the latent image is visualized, the amount of bending of scanning lines appears as a color shift and deteriorates an image quality. Moreover, by the oblique incidence, the light beam is twisted and incident on a scanning lens, thereby increasing wave front aberration and causing remarkable deterioration of an optical performance particularly in a peripheral image height. This, results is thickening of a beam spot diameter, which hinders forming of an image of high quality. Further, in the oblique incidence, since the light beams from the light source are caused to be incident towards the axis of rotation of the polygon mirror, if the light source is disposed in a position overlapping an optical axis of the scanning lens in the main scanning direction, the angle of oblique incidence becomes wider to avoid interference with the scanning lens.
As a method of correcting the substantial bending of scanning lines, which is peculiar to the oblique incidence, a method that includes in the scanning imaging optical system, a lens that has a lens surface in which a proper inclination of the lens surface in a secondary scanning cross section is changed in the main scanning direction to correct the bending of the scanning lines (see, for example, Japanese Patent Application Laid-open Publication No. H11-14932) or a method that includes in the scanning imaging optical system, a correcting and reflecting surface that has a reflecting surface in which the proper inclination of the reflecting surface in the secondary scanning cross section is changed in the main scanning direction to correct the bending of the scanning lines have been proposed (see, for example, Japanese Patent Application Laid-open Publication No. H11-38348).
A problem in the oblique incidence at present is that, there is a tendency of substantial deterioration of the wave front aberration near the peripheral image height, in other words near both edges of the scanning lines due to skew rays. If there is such a wave front aberration, the spot diameter of an optical spot in the peripheral image height becomes bigger. If this problem cannot be solved, an optical scanning of high density which has been much sought after recently cannot be realized. In optical scanning units disclosed in Japanese Patent Application Laid-open Publication No. H11-14932 and Japanese Patent Application Laid-open Publication No. H11-38348, although the substantial bending of the scanning lines which is peculiar to the oblique incident has been corrected quite satisfactorily, the correction of the wave front aberration cannot be said to be sufficient.
The deterioration of the wave front aberration and the bending of the scanning lines can be said to be the problems in the oblique incidence. An optical scanning unit that can correct these two satisfactorily, including a plurality of rotating asymmetrical lenses in the scanning imaging optical system where a shape of a main line that connects vertices of a sub line of lens surfaces of the rotating asymmetrical lenses is bent in the secondary scanning direction, has been proposed (see, for example, Japanese Patent Application Laid-open Publication No. H10-73778).
However, a lens that has a lens surface, which is bent in the secondary scanning direction the shape of the main line that connects the vertices of the sub line that is used in the invention disclosed in the Japanese Patent Application Laid-open Publication No. H10-73778 is required to have greater lens width in the secondary scanning direction since the main line is bent. Particularly, in a lens surface having a bigger curvature, an amount of bending of the main line for correcting the bend in the scanning lines, the lens width is to be increased substantially. Moreover, since the lens has a curvature in the secondary scanning direction, when the lens rotates around an optical axis, there is more deterioration of the wave front aberration. If the main line is caused to be bent in a case of a toric surface that has a bend of the secondary scanning direction different from the main scanning direction, a shape in the main scanning direction changes according to height of the secondary scanning direction, and if a position of incidence of the light beam in the secondary scanning direction is shifted due to a change in temperature and an error in assembling of optical elements, there is a substantial change in magnification error. As a result, in the color image forming apparatus, a beam spot position between each of the colors is shifted, thereby resulting in a color shift.
Next, reduction in the color shift to obtain a high quality image is taken into consideration. To solve the problems described so far and to improve scanning characteristics, a use of a special surface that is typified by an aspheric surface, has been generalized. Optical elements made of resin that enables to form such a surface easily and which are low cost have been used a lot. Particularly, in the tandem image forming apparatus, since the number of the optical elements used is more, the cost is reduced substantially by using the optical elements made of resin.
However, if the optical elements made of resin are used in the optical scanning unit, since the resins have a coefficient of thermal expansion greater than that of glass, there is a substantial amount of change in a shape due to a change in temperature, resulting in a change in optical characteristics of the optical elements made of resin. Since the optical scanning unit includes a light deflector such as a polygon mirror that generates a substantial amount of heat, when the temperature in an optical box rises due to the light deflector, because of an air flow caused by the rotation of the polygon mirror, and a difference in the shape inside the optical box, the heat is not transmitted uniformly and the temperature in the optical box is not uniform everywhere. Moreover, regarding the scanning lens, due to a difference in transmission of heat, a difference in shape of the lens, and a difference in installation area in the optical box, the temperature change is not uniform and there is a difference in temperature where the scanning lens is disposed.
In the tandem image forming apparatus, light beams heading towards each of the photosensitive drums pass through different scanning lenses, and due to the temperature distribution in the optical box that holds the scanning lenses, there is a different temperature distribution between each of the scanning lenses, due to which the change in the shape and a change in the refractive index of the scanning lens is non-uniform. Therefore, an amount of change in the length of the scanning lines and a change in a constant velocity is different for each of the photosensitive drums. When latent images formed on the photosensitive drums by such an optical scanning are visualized by a developing unit that uses developers of different colors such as yellow, magenta, cyan, and black, then the visualized images are transferred by superimposing the images one after another on the same recording paper and fixed to obtain a color image, there is an occurrence of the so called color shift. Particularly when a scanning lens made of resin is used as a scanning lens that is nearest to the light deflector such as a polygon mirror, which generates substantial amount of heat in the optical box, there is a substantial change in the optical characteristics.
Moreover, when images are printed out continuously, particularly when the number of images printed out is more, due to the heat generated by the light deflector, the temperature in the unit (temperature in the optical box) goes on increasing. Therefore, the temperature distribution of each lens changes, resulting in the color shift in the images output and an amount of the color shift goes on changing. As a result, there is a change in a hue due to the color shift in the first image and the last image printed out.
As a method to cope with the change in the scanning length, a method in which a light receiving unit is disposed on a writing-start side and a writing-end side in the main scanning direction, and an image frequency of each light beam is adjusted based on a difference in time of receiving of light by each of the light receiving units is disclosed in Japanese Patent Application Laid-open Publication No. H9-58053. If an attempt is made to use this method in the tandem image forming apparatus that uses a light deflector jointly for the plurality of scanning surfaces, a space is required for disposing the light receiving units on the writing-end edge side and it is even more difficult to secure effective writing width. Moreover, in the method in which the light receiving unit is disposed on the writing-start side and the writing-end side, and the image frequency of each light beam is adjusted based on the difference in time of receiving of light by each of the light receiving units, the length of the scanning lines in each photosensitive drum can be corrected, but the change in the constant velocity caused due to the temperature distribution of each scanning lens cannot be corrected. Therefore, even if a dot position in a writing-start position and a writing-end position in the main scanning direction is corrected in each of the photosensitive drums, the dot in between the main scanning direction does not coincide and there is an occurrence of color shift.
In the tandem optical scanning unit, to solve this problem, in many cases where the scanning lens that is nearest to the light deflector like a polygon mirror, which generates heat, is made of glass, and the cost goes up substantially as compared to that when the scanning lens made of resin is used.
The following are the characteristics and issues of the tandem image forming apparatuses according to items 1 to 3.                (1) A method of using a plurality of optical scanning units corresponding to the plurality of photosensitive drums—Being a different optical scanning unit, due to a difference in a shape of the optical elements and due to environments of different temperature and moisture, there is a relative shift in a beam position in each scanning surface and it is susceptible to occurrence of color shift.        (2) A method of scanning in opposite directions in which the light beams are incident from both sides of the light deflector and the deflection scanning is performed by dividing into two directions—The size of the optical scanning unit can be reduced easily as compared to that in item (1) and a high cost polygon scanner can be used jointly for four colors, thereby proving it to be favorable for cost reduction. On the other hand, in the system for the scanning in opposite directions with the polygon at the center, due to the difference in the shape of the optical elements and due to the environments of different temperature and moisture, there is a relative shift in a beam position in each scanning surface and it is susceptible to the occurrence of color shift.        (3) A method of one side scanning in which light beams are incident from one side of the light deflector and the deflection scanning is performed in one direction—Since each light beam corresponding to each color (Yellow (Y), Magenta (M), Cyan (C), and Black (K)) passes jointly through a scanning lens (a first lens) in common that has power mainly in the main scanning direction, the difference in the shape of optical elements is not much and the environments of the temperature and moisture are almost the same. Therefore, the change in the relative position of the beam spot (in other words, color shift) in the main scanning direction of each color is difficult to occur. On the other hand, the following issues are there.        (1) In a case of the optical element through which the light beams corresponding to each color (Y, M, C, and K) pass jointly, since the lens height in a direction corresponding to the secondary scanning increases, it is difficult to compensate for a difference in profile irregularity of each light beam, thereby reducing the size of a diameter of the beam spot. Moreover, it is necessary to reduce an increased cost resulted due to an increase in formation time caused by thickening.        (2) Since the light beam corresponding to each color (Y, M, C, and K) is incident on the same deflecting mirror surface, there is an increase in the thickness of the polygon mirror resulting in an increase in the size of the light deflector and the apparatus. This also results in a wind loss, thereby causing noise, increased power consumption, and deterioration of durability.        (3) To separate each light beam corresponding to each color (Y, M, C, and K), the structure is such that each light beam is allowed to be incident obliquely on the light deflector mirror surface. Therefore, due to skew of the light beam caused by the deflection at the polygon mirror, the wave front aberration is susceptible to deterioration and there is a decrease in the size of the beam spot diameter. Moreover, due to the bending of the scanning lines on an image carrier (photosensitive drum), an adjustment to correct the bending of scanning lines is indispensable.        